1. Field of the Invention
This invention relates to a polypropylene multi-layer film having an improved low-temperature heat sealing characteristic. More particularly, it relates to a polypropylene multi-layer film having improved heat sealing property, resistance to solvent extraction and other film properties, and particularly suited to food packaging, which film comprises a substrate layer formed of crystalline polypropylene and a composition laminated thereon, said composition comprising a general purpose crystalline propylene-.alpha.-olefin copolymer having unsatisfactory low-temperature heat sealing property and a relatively small amount of a specified crystalline propylene-butene-1 copolymer incorporated thereto.
2. Description of the Prior Art
Stretched film of crystalline polypropylene is excellent in mechanical strength such as tensile strength, rigidity and low-temperature resistance, optical properties such as gloss and transparency, and food hygienic properties such as innoxiousness and odorlessness, and hence is widely used in the field of packaging including food packaging.
However, said polypropylene stretched film used as a single layer has a defect in that it has an extremely poor heat sealing property and, if forcibly heat sealed, undergoes thermal shrinkage, resulting in deterioration of the appearance of the product, so that the heat sealing is virtually impossible.
Various attempts have been made to obviate the above-mentioned defect. A widely used method among them is to use a multi-layer film formed by laminating on a substrate of polypropylene a resin layer having a low-temperature heat sealing property superior to that of the substrate. In such a method, multi-layer film formed by laminating, as a resin of superior heat sealing property, polyethylene, ethylene-vinyl acetate copolymer, or the like has a defect in that though it is excellent in heat sealing property it shows deteriorated transparency and scratch resistance. On the other hand, for the purpose of obviating such a defect, there have been proposed multi-layer films having laminated therein propylene-ethylene copolymer of an ethylene content of about 0.5 to 5% by weight [Japanese Patent Application Kokoku (Post-Exam. Publication) No. 31478/71], propylene-ethylene-butene-1 terpolymer [Japanese Patent Application Kokai (Laid-open) No. 35487/74], and propylene-.alpha.-olefin copolymer having a content of .alpha.-olefin of 4 or more carbon atoms of 5 to 20% by weight [Japanese Patent Application Kokai (Laid-open) No.128781/75]. However, although these multi-layer films are excellent in transparency and scratch resistance, their low-temperature heat sealing propertry is not sufficiently good, and resultantly their aptitude for high speed packaging, etc. is unsatisfactory.
Accordingly, a number of attempts have been made to improve the low-temperature heat realing property while maintaining the good characteristics of stretched polypropylene film such as transparency and scratch resistance as much as possible. For example, Japanese Patent Application Kokai (Laid-open) No. 14223/80 discloses "a stretched composite polypropylene film which comprises a stretched crystalline polypropylene layer and a composition (A) laminated on at least one side of said layer, said composition (A) comprising 3 to 97% of an ethylene-propylene random copolymer having an ethylene content of 1 to 10% by weight, and 3 to 97% by weight of a propylene-.alpha.-olefin or propylene-.alpha.-olefinethylene copolymer having a propylene content of 65 to 99% by weight, a content of .alpha.-olefin of 4 to 8 carbon atoms of 1 to 30% by weight and an ethylene content of 0 to 5% by weight". Japanese Patent Application Kokai (Laid-open) No. 58861/81 discloses "a polypropylene composite film which comprises a substrate layer formed of an isotactic polypropylene layer and a polyolefin composition laminated on at least one side of the said substrate layer, said polyolefin composition comprising 10 to 40% by weight of a propylene-butene-1 random copolymer having a propylene content of 55 to 80% by weight and a heat of crystal fusion of 20 to 80 Joule/g as determined by thermal analysis on a differential scanning calorimeter, and 90 to 60% by weight of a crystalline propylene-.alpha.-olefin random copolymer having a propylene content of 99 to 93% by weight.
However, these attempts of improving the low-temperature heat sealing property by incorporation of crystalline propylene-butene-1 copolymer disclosed above all give, to the best of the knowledge of present inventors, only unsatisfactory products because they have defects in that they are unsatisfactory in low-temperature heat sealing property, or, when their low-temperature heat sealing property is on an approximately satisfactory level, they show deterioration in properties important to packaging film, such as blocking resistance and solvent resistance. Thus, there have been serious problems in that when the amount of soft resin, which is a crystalline propylene-butene-1 copolymer, added for improving the low-temperature heat sealing property is a relatively small value of 30% by weight or less, although scratch resistance, etc. of the resultant film are usually maintained on a good level, its low-temperature heat sealing property is poor; whereas when the added amount of soft resin is 40% by weight or more, although the low-temperature heat sealing property is satisfied in the main, the film is poor in important properties including scratch resistance.